This invention relates generally to methods and apparatus for sterilizing articles, and more particularly, to methods and apparatus for sterilizing dental and other medical instruments, such as dental handpieces.
The need for effective sterilization of dental instruments, such as dental handpieces, is more important today than ever before due to the realization of the significant potential for infection via dental procedures and the increase in the rate of transmission of serious diseases by blood and saliva. However, from a practical viewpoint, not only must the sterilization procedure be effective, it must also be rapid, i.e., have a short turn-around or cycle time. Thus, it will be understood that the quantity of any one instrument that a practitioner or institution must purchase and have available for use will depend on the frequency of treatments requiring that instrument and on the turn-around time required to sterilize the instrument. In the case of dental handpieces, which generally are required for most treatments performed in the dental operatory, an extended sterilization cycle means that a larger inventory of available handpieces must be maintained. The high cost of dental handpieces, on the other hand, often limits the number of available handpieces, which in turn may result in hasty and therefore ineffective attempts at, or even dispensing with, lengthy sterilization procedures. This of course is unacceptable.
Heat sterilization methods, such as steam heat (autoclave), dry heat, and chemical vapor, are generally preferred over cold immersion methods, which are generally not effective unless glutaraldehyde is used and the instrument is immersed for 7-10 hours. Chemical vapor sterilization has the advantages of minimal corrosion of burs and other sharp instruments (steam sterilization or immersion in most liquid disinfectants produces dullness and rusting) and a cycle time which is relatively short compared to cold sterilization techniques.
One type of chemical vapor sterilizer which is in commercial use (available from MDT Corporation of Gardena, Calif. under the designation Harvey Chemiclave) comprises a sterilizer which uses moderate heat (about 270.degree. F.), pressure (about 20 psi) and a special solution composed primarily of 3A alcohol (about 80%) and water (about 9%) with small amounts of acetone, ketone and formaldehyde. The sterilization time is about 20 minutes after proper pressure is reached. The length of time required to reach pressure depends on the size of the load. On the other hand, the purchase price of this arrangement is higher than steam and dry heat processors. Additional drawbacks are that relatively large quantities of sterilizing solution are required, and odor and minor irritation of eyes, nose and throat from chemical vapors often accompanies use, the sterilizing chamber must be cleaned on a frequent basis, the instruments should be wrapped to preserve sterility, and the simultaneous sterilization of a plurality of instruments results in the possibility of cross-contamination. Moreover, the sterilization cycle time, although less than the time required for the sterilization by solution immersion, is still relatively long, essentially because of the relative great chamber volume.
U.S. Pat. No. 4,400,357 issued Aug. 23, 1983 to Hohmann, discloses an arrangement for chemical vapor sterilization of articles, such as dental handpieces, which would appear to overcome some of the above-mentioned problems. The patent discloses an arrangement in which the article to be sterilized is situated in an enlarged portion of a rigid vessel. A liquid reaction agent is charged into a narrow portion of the vessel which is in communication with the enlarged article-containing vessel portion. The liquid reaction agent is heated to produce a vapor which flows into the first vessel portion to sterilize the article. The first vessel portion may be designed to accept only a single article in which case the amount of liquid reaction agent required to generate the vapor is relatively small which in turn reduces the heating time required for vaporization and the overall sterilization cycle time. The patent suggests that the means for heating the liquid reaction agent may comprise a microwave radiator. In this case, the vessel is situated such that only the narrow liquid-containing vessel portion is subjected to the microwave radiation while the article to be sterilized is kept outside the radiation field which, the patent notes, avoids the formation of spark gaps at border surfaces and seams of the article which cause surface destruction. In any event, although microwave radiation is known to have beneficial sterilizing effects, the patent notes that microwave radiation will not penetrate into the seams and crevises of the article and not kill micro-organisms situated therein. Although possibly reducing the time required for sterilization, the arrangement proposed in the patent has various drawbacks which have apparently prevented adoption and commercialization of this arrangement. For example, it requires a complicated, specially designed microwave generator adapted for positioning the vessel with only the liquid reaction agent-containing portion in the radiation field of the microwave generator with the article-containing vessel portion outside the radiation field. It requires a specially designed rigid vessel which either must be cleaned after each use to avoid cross-contamination or discarded at significant expense. Moreover, the sterilizing effect of the microwave radiation is not utilized since the microwave energy is used only for vaporizing the liquid reaction agent.
A good discussion of the sterilization of articles, such as dental instruments, by microwave radiation is set forth in U.S. Pat. No. 3,753,651 issued Aug. 21, 1973 to Boucher. Briefly, it is noted that sterilization by microwave radiation is due to both thermal effects, such as microwave induced heat, and non-thermal effects, which the patent suggests may affect a metabolic system distinct from that of thermal energy. It is disclosed that improved surface sterilization results are obtained when the articles are subjected to microwave radiation while situated in a humid atmosphere, i.e., an atmosphere having a relative humidity of least 50% or supersaturated with water or saline solution. To this end, the articles to be sterilized are placed on trays which are situated in a rigid, microwave-transparent container having a known volume, along with a quantity of water or saline solution determined by the container volume so as to be sufficient when vaporized to increase the humidity of the atmosphere within the container to the desired value. After placing the articles to be sterilized and the water or saline solution into the container, the container is sealed with a lid and then placed within the cavity of a microwave generator and subjected to microwave radiation. The electro-magnetic energy penetrates through the container walls to evaporate the water or saline solution to produce the desired humidity, and at the same time, proceeds to sterilize the surface of the article by the thermal and non-thermal effects discussed above. It is indicated that this procedure results in reduced cycle time for effective sterilization compared to dry heat or steam sterilizing methods and that the localized arcing (sparking) which usually occurs when metallic objects are irradiated by microwave radiation is practically eliminated in the moist atmosphere.
The patent also points out that the container can be filled with any gas to constitute the atmosphere to be humidified. For example, it is suggested that a gas or vapor sterilant can be introduced into the container through valves provided in the container walls to take advantage of their chemical sterilizing effects, although care should be taken to avoid heating the article being sterilized to a point where it reaches the ignition or explosion point of the gas,
The arrangement proposed in U.S. Pat. No. 3,753,651 has drawbacks which have apparently prevented it from being adopted on a practical basis. For example, as noted in the above-discussed U.S. Pat. No. 4,400,357, only surface sterilization is achieved by microwave irradiation and micro-organisms present on surfaces located within the seams and crevices of the article will not be killed, especially if blood and salivary protein are deposited on those surfaces. This is true regardless of whether the container is initially filled with a gas sterilant as suggested in the patent. The procedure requires a specially designed gas-tight rigid container having a known, fixed volume. The container must be sterilized after each use or discarded, in which case considerable expense is incurred especially where valves are provided in the container walls as discussed above. To provide a truly gas-tight condition, it is necessary to use materials, such as for gaskets and the like, which are not entirely transparent to microwaves. Moveover, the relatively large volume of the container which is necessary to accommodate the articles to be sterilized in turn requires a relatively large volume of water or saline solution to achieve the desired humidity. This results in an increase in the time required for the evaporation of the water or saline solution thereby increasing the overall sterilization cycle time.